1. Field of the Invention
The present invention relates generally to optical pickup units and information recording apparatuses, and more particularly to an optical pickup unit recording information on and reproducing information from the recording surface of a multilayer information recording medium using the differential push-pull method detecting and controlling the position of an emitted light beam, and an information recording apparatus using the same.
2. Description of the Related Art
In information recording apparatuses, a main light beam is emitted from a light source onto the recording surface of an information recording medium having guide grooves (information tracks) on which the information is recorded, so that information is recorded on the information recording medium or information is reproduced from the information recording medium based on light reflected from the recording surface. The information recording apparatus includes an optical pickup unit for emitting the main light beam on the recording surface of the information recording medium and receiving light reflected from the recording surface.
Further, as shown in FIG. 1, the optical pickup unit includes a light source 11, an optical system including an objective lens 16, and a divided light receiving element 19. The optical system further includes a coupling lens 12, a beam splitter 14, and a deflection mirror 15. The optical system guides a light beam emitted from the light source 11 to the recording surface of an information recording medium 18 and guides a returning light beam reflected from the recording surface to a predetermined light-receiving position, where the divided light receiving element 19 is disposed. The divided light receiving element 19 outputs not only the reproduced information of data recorded on the recording surface but also signals including information necessary for controlling the positions of the optical pickup unit itself and the objective lens 16. These signals are fed back so as to control their positions, so that driving control is performed by a lens actuator 17.
In order to accurately record data at a predetermined position on the recording surface and reproduce data recorded at a predetermined position on the recording surface, a main light beam should be emitted accurately onto the predetermined position of the recording surface. This makes it necessary to detect the position of the emitted main light beam with accuracy. As a result, there have been proposed various methods for detecting the position of the emitted main light beam on the recording surface using a returning light beam reflected from the recording surface, some of which have been put to practical use.
As a method of detecting the position of the main light beam on the recording surface from a returning light beam reflected from the recording surface, a so-called push-pull method is widely used.
As shown in FIG. 2, according to the push-pull method, a light beam emitted from a light source is split into one main light beam 1 and two sub light beams 2, so that the main light beam 1 and each of the sub light beams 2 are offset from each other radially on the recording surface by half a track pitch. The returning light beams of the main light beam 1 and the two sub light beams 2 reflected from the recording surface are received by three divided light receiving elements 19a through 19c, respectively, shown in FIG. 3. Each of the divided light receiving elements 19a through 19c is divided into two parts. A push-pull signal is obtained from each of the divided light receiving elements 19a through 19c. A tracking error signal (DPP) of Eq. (1) is obtained from the difference between the push-pull signal of the main light beam 1 (MPP) and the sum of the sub push-pull signals of the two sub light beams 2 (SPP) as follows:DPP=MPP−K(SPP),  (1)where K is an integer. Based on the thus detected tracking error signal, so-called tracking control that drives the objective lens in the radial directions of the information recording medium is performed.
At the time of recording information on the information recording medium, the reflectivity of the guide grooves varies. Therefore, as shown in FIG. 4A, in recording information on the information recording medium in a direction from the center to the periphery thereof, disposing a first sub light beam 2a and a second sub light beam 2b so that the first sub light beam 2a leads the main light beam 1 on its inner side and the second sub light beam 2b trails the main light beam 1 on its outer side in the optical disk scanning direction causes an offset in the differential push-pull signal due to the difference in reflectivity between an unrecorded part 4 and a recorded part 5. Therefore, according to Japanese Laid-Open Patent Application No. 2004-164720, as shown in FIG. 4B, the first and second sub light beams 2a and 2b are disposed so that the first sub light beam 2a leads the main light beam 1 on its outer side and the second sub light beam 2b trails the main light beam 1 on its inner side in the optical disk scanning direction, thereby eliminating the effect of the difference in reflectivity in each of the first and second sub light beams 2a and 2b and thus reducing an offset in the differential push-pull signal.
Multilayer information recording media are employed as means for increasing the capacity of information recording media. Of the multilayer information recording media, those put to practical use at present include DVD+R double layer (DL) disks and DVD-ROM double layer (DL) disks.
FIG. 5 shows a structure of the DVD+R DL disk. Referring to FIG. 5, the DVD+R DL disk includes a substrate 0 layer, a dye 0 layer, a translucent film, an intermediate layer, a dye 1 layer, a reflection film, and a substrate 1 layer that are stacked in the order described from the light incidence (objective lens) side.
Signal information is recorded as variations in refractive index on a guide groove from the dye 0 layer to the dye 1 layer. In general, polycarbonate may be used for the substrate 0 layer and the substrate 1 layer, and a UV cure or thermosetting resin may be used for the intermediate layer. Silicon, silver, or aluminum may be used for the translucent film, and silver or aluminum may be used for the reflection film.
The guide groove is formed spirally on each recording surface. The direction of the spiral differs between an L0 layer on the objective lens side and an L1 layer on the other side of the L0 layer from the objective lens. This structure where the direction of the spiral of the guide groove differs between the L0 layer and the L1 layer is referred to as “Opposite Track Path (OTP).”
In an OTP double layer disk, information is recorded in a direction from the center to the periphery of the disk on the L0 layer and in the opposite direction, from the periphery to the center, on the L1 layer.
For related techniques, reference may be made to Japanese Laid-Open Patent Application Nos. 2003-196860 and 2002-015442.
FIG. 6A shows a conventional disposition of the main light beam 1 and the sub light beams 2a and 2b in the case of recording information on the DVD+R DL disk. In recording information in the L0 layer, an excellent push-pull signal can be obtained with the above-described technique disclosed in Japanese Laid-Open Patent Application No. 2004-164720 since there is no difference in reflectivity in each of the sub light beams 2a and 2b. 
However, in recording information in the L1 layer, in which information is recorded in a direction from the periphery to the center of the disk, the area of the recorded part 5 is reversed as shown in FIG. 6B. As a result, a difference in reflectivity is caused in each of the sub light beams 2a and 2b, so that an offset is caused in the push-pull signal. That is, in the case of recording information on a multilayer information recording medium having an OTP structure, the technique of Japanese Laid-Open Patent Application No. 2004-164720 has a problem in that an offset is caused in one of the recording layers of the recording medium.